a unique set of HLA proteins on a person’s white blood cells. HLA types differ among people just like blood types differ among people. HLA testing is used to determine a person’s HLA type. HLA testing is done before a type of treatment that transfers blood stem cells from another person to the patient. It’s very important that their HLA types are a near-perfect match for this treatment to work. This is because the HLA type affects how the body responds to foreign substances